Advanced semiconductor fabrication is employing increasing vertical circuit integration as designers continue to strive for circuit density maximization. Such typically includes multi-level metallization. Electrical interconnect techniques typically require electrical connection between metal layers or other conductive layers which are present at different elevations in the substrate. Such interconnecting is typically conducted, in part, by etching a contact opening through insulating material to the lower elevation metal layer or conductive region. Increased circuit density has resulted in narrower and deeper electrical contact openings between layers within the substrate. Adequate contact coverage within these deep and narrow contacts continues to challenge the designer in assuring adequate electrical connection between different elevation areas within the substrate.
This invention arose principally out of problems associated with tungsten plug formation in contacts formed through silicon dioxide insulating layers. The problem is best understood with reference to the accompanying FIGS. 1 and 2. There illustrated is a semiconductor wafer fragment 10 comprised of a bulk substrate 12 and an overlying silicon dioxide layer 14 such as borophosphosilicate glass (BPSG). Bulk substrate 12 includes a dopant diffusion/active region 16 to which electrical connection is to be made. A contact opening 18 is provided through BPSG layer 14 to active area 16.
A thin layer 20 of titanium is deposited atop the wafer to within contact opening 18. Titanium layer 20 is provided to function as a silicide formation layer at the base of contact 18 for reducing resistance. An undesired oxide layer (not shown) also typically forms atop diffusion region 16. The deposited elemental titanium also functions to break-up this undesired oxide and thereafter form a titanium silicide with the silicon of substrate 12 to reduce contact resistance between active area 16 and a subsequently deposited plug filling tungsten. Additionally, titanium layer 20 functions as an adhesion/nucleation layer for the subsequently deposited tungsten. Tungsten does not readily deposit over silicon dioxide and exposed silicon substrate, and the intervening titanium layer 20 facilitates deposition and adhesion of tungsten thereto.
Titanium layer 20 is typically deposited by sputter deposition, and undesirably results in formation of contact projecting cusps 22. This results in a back or re-entrant angle 24 being formed relative to contact opening 18. A layer 26 of tungsten is subsequently deposited, with the intent being to completely fill the remaining volume of contact opening 18. Unfortunately, an undesired keyhole 28 typically forms, leaving a void within contact 18.
Referring to FIG. 2, layers 26 and 20 are subsequently etched back by dry etch or chemical-mechanical polishing to form a contact-filling plug 30. Undesirably, this typically opens-up the upper end of keyhole 28. The etch back also undesirably etches titanium layer 20 and forms sharp edge "fangs" or "divots" 32. Even if titanium layer 20 were not present, the fangs would also typically be produced. With respect to the described example, this could occur by chemical attack of the oxide and titanium during the tungsten etch and by a typical subsequent HF dipping step.
Regardless, this can lead to subsequent problems associated with the Metal 1 layer deposition, as shown in FIG. 3. The Metal 1 layer typically comprises a composite thin titanium glue layer 34, a bulk conductive Al-Cu alloy layer 36, and an antireflective titanium nitride coating layer 38. Other composite layers are of course possible. Regardless, fangs 32 typically preclude or hinder deposition of the composite Metal 1 layer. Specifically, glue layer 34 does not fill fangs 32 and alloy layer 36 produces cusps which hinder deposition even atop plug 30. Glue layer 34 and coating layer 38 do contact the top of plug 30 but, in the extreme case, adequate electrical connection is not made between the composite Metal 1 layer and tungsten plug 30. This problem is especially pronounced with tungsten plug formation, but can also occur with other electrically conductive plug filling materials such as other metals, metal alloys and polysilicon.
It would be desirable to overcome these and other problems associated with formation of electrically conductive contact plugs.